I don't know if it's haraam or not, but I wouldn't recommend it for a number of reasons.

1. I think of it like a person walking into a bar and saying they won't drink, but will give da'wah instead. You're walking into an area on purpose which is designed against Islaam, and for spreading fisq and kufr, with like-minded people in order to spread Islam? I don't know, I would hate to go into that sort of situation knowingly.

2. This is something a brother once said about why he doesnt like to debate about Islam: he would fear that perhaps he did not have the knowledge to be able to convey Islam properly, or present the correct argument, and people would mistake his lacking (in ability and knowledge) as a lacking in Islam. It takes a certain level of knowledge and ability to do these types of things anyway, and perhaps you have that, I don't know - but generally for the common person, sometimes this isn't a good course of action.

3. Many times those who walk in with good intentions, walk out in doubt or worse than it. I've seen this effect in people who overestimated themselves, but wanted to do good so walked in confidently to these sites, yet due to their weakness in Imaan soon found themselves questioning their own faith, their own understanding, slowly finding themselves corrupting, compromising, adjusting their speech to fit their 'dawah' - and in the worst case scenarios, are perhaps won over by the counter-arguments. A'uthobillah.

4. Those who frequent these sites are deaf, dumb and blind usually. And personally, I believe my time is much better spent even talking to a wall than these people. You need to approach people on a certain level - but some will never allow you to do that, they're bigoted, racist, prejudiced and trashy, and this is something ingrained in them so deep, that your words will never even be considered. And thus this will be a waste of time, and may cause you to waste precious moments that you could spend otherwise doing da'wah through productively - such as in a local university, through your masjid - etc...

This is just my take, especially with sites such as you've indicated that are, 'anti-Islam'.

PS: I just noticed you addressed this to the brothers in particular...sorry about butting in.

Last week I thought about joining such a forum, but then I had almost the same concerns as you mentioned and therefore I didn't join.
My intention was actually not to make da'wah to them, but to respond to some of their lies, because I think there are also many neutral non-muslims who read this particular website (it's not in English) and get a wrong image of Islam because of that.

Don't know of anything specifically pronouncing it as haraam, but I would tend to question how constructive it is when we go into an expressly anti-Muslim environment and attempt to defend the Deen. For starters, if we were to try and refute every lie told on the internet about Islam, we might never get up from the computer again. Second, when we do go into such an environment, the outcome is usually pre-determined anyway by the nature of the environment (in this case the forum you mentioned); so even if you were to bring an airtight argument showing them their error, they may still refuse to accept it out of sheer vanity, stupidity, stubborness, etc. Thirdly, we risk out own Imaan, as sister Prolix pointed out (wa A3udh biLlahi min ash-shaytaan ar-rajim). Fourthly, if our 'ilm is lacking, we risk misrepresenting the Deen. Fifthly, if our motives are not pure, we may find that we set out to defend the Deen, but ended up struggling in vain to defend nothing more than our own vanity. Wa Allahu A'lim.

And there's this to consider as well:

“I have never debated with a knowledgeable person but beaten him, and I have never debated with an ignorant person but been beaten by him.” - Imam al-Shafi`i (Rahimahu Allah)

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May Allah (subhanaHu wa ta'aala) make us all into effective defenders of the Deen against its enemies...